The removal of volatile components from a polymer solution is one of the final process steps in the production of many polymers. The volatile components to be removed may, for example, be solvents, unpolymerized monomers or oligomers formed during the reaction. Removal of the residual volatile components from the polymers often involves heating, which may lead to undesired effects such as impairment of the intrinsic color, crosslinking or de-polymerization with reformation of monomers or oligomers. It is always of interest to combine small losses of quality with good degassing results, since the residual volatile components may impair the material properties and produce undesired odors, and there are also the health considerations since many of these substances are toxic.
Various methods for removing the volatile components from the polymer solution are known. Removal of residual monomers by chemical means is described, for example, in EP 0 768 337 A1. The removal is carried out by adding CH-acidic organic compounds. The chemical conversion of residual monomers likewise leads to products with undesired ecological relevance, which makes it significantly more difficult to use the products in practice. This method also cannot be used for removing residual solvents.
The method for reducing residual monomers with unsaturated fatty acids according to U.S. Pat. No. 4,215,024 suffers from the same deficiencies.
Another known method describes the reduction of residual monomers by treating the molding compositions with electron beams, as described in DE 28 43 292 A1. The method is, however, much too expensive to be implemented on an industrial scale. A method described in EP 0 798 314 A1 for removing residual volatile components by injecting supercritical solvents into the polymer melt, with a subsequent expansion, is found to be just as expensive.
Conventional methods are based on the removal of residual monomers and solvents with the aid of mechanically supported systems. For example, extruders (U.S. Pat. No. 4,423,960), degassing centrifuges (U.S. Pat. No. 4,940,472) or thin-film evaporators (DE 19 25 063 A1) are used.
All these methods have the disadvantage of requiring heavy moving parts in devices. This leads to processes which are cost-intensive and susceptible to malfunction and wear. The large mechanical energy input in such a process also leads to high temperatures, which in turn promote product damage. The mechanical energy is usually generated from electrical energy, which leads to high costs and a greater burden on the environment compared to the use of primary energy.
DE 100 31 766 A1 proposes a two-stage continuous method for degassing styrene copolymers, in which the polymer concentration is brought to more than 99.8 wt. % in a first stage in a shell-and-tube heat exchanger by evaporating volatile components with simultaneous energy input, and the final concentration is reached in a second stage, an extrusion evaporator, without intermediate overheating. Disadvantages of this method are the long residence time at high temperature in the bottom of the two stages, which may lead to undesired product discoloration, and in the long residence time in the tube evaporator of the first stage, in which heating takes place with a high wall temperature in the presence of residual monomers.
U.S. Pat. No. 4,699,976 describes a two-stage continuous method for degassing styrene polymers containing rubber. This method uses two degassing stages, which are equipped with shell-and-tube heat exchangers. In the first stage, the polymer solution is concentrated to a residual volatile-component content of between 3% and 15%. In the second stage, evaporation takes place to the final concentration. Foaming takes place inside the tubes. Disadvantages in this case are the long residence time with exposure to heat and in the presence of residual monomers, which is due to the use of conventional shell-and-tube equipment in the second stage.
EP 0 749 343 B1 describes an apparatus and a method in which the heat transfer to the polymer solution takes place using a specially formed plate heat exchanger. The product in this case emerges horizontally from horizontal slits arranged above one another. Disadvantages of this arrangement are that different foamed polymer extrudates combine together, so that the accessibility of the vacuum to the polymer extrudates is made more difficult. Such an arrangement will therefore have high residual volatile-component contents, which is undesired.
“Neue Mischverfahren mit geringem Energiebedarf für Polymerherstellung und-aufbereitung”, Chemische Industrie (1985) 37(7), pages 473 to 476 describes a method with which an entrainer is mixed with the polymer prior to the last stage, before the product is brought into the last stage, a degassing container. Primarily, as is familiar to the person skilled in the art, inert gases such as nitrogen or carbon dioxide, or alternatively water, are used as entrainers. Both methods have disadvantages. Inert gases reduce the power of the coolers in which the volatile components are intended to condense, and increase the volume to be delivered by the vacuum system, so that the method becomes more expensive. The use of water is disadvantageous because the temperature of the condensers must then be restricted to above 0° C. in order to prevent freezing, so that the power of the condensation system is restricted, which in turn needs to be compensated for by a larger and more expensive vacuum pump.